The invention relates to a method and a device for controlling a hydraulically operated power wrench comprising a pivotable lever pivoted by a piston-cylinder unit and provided with a ratchet arrangement for stepwise rotation of a ratchet bushing by advancing and returning strokes and further comprising a drive unit with a motor-driven pump.
US 2005/0210872 A1 discloses such a method and a device in which the change of the hydraulic pressure is measured in time intervals and the working process is terminated when the increase of hydraulic pressure for a load stroke within a predetermined time is smaller than a predetermined limit value, wherein the pressure loading cycles cause alternating advancing strokes and return strokes and wherein during the load stroke the temporal change of the hydraulic pressure is measured in time intervals and switching to a return stroke is realized when in at least one of the time intervals the pressure increase is greater than in at least one of the preceding intervals of the working process.
In order to carry out the method as described, the operator first sets a nominal pressure which is that pressure that determines the torque of the screw to be turned and at which pressure the working process can be terminated. The characteristic pressure/time curve of the respective pressure device is determined prior to operation and saved. The slope of the pressure increase curve is determined in that the pressure is measured in time intervals of 10 ms. Since the time intervals have all the same duration, for determination of the differential quotient y=dP/dt the pressure difference dP occurring within a time interval is evaluated and saved in digital form. When the terminal pressure is reached the pressure remains constant and the differential quotient dP/dt=0.
In this method it is not only necessary to carry out before the actual working process a calibration but it is also required to carry out continuously pressure measurements in time intervals of 10 ms in order to determine the pressure increase that serves as a criterion for switching from advancing stroke to return stroke and for switching off upon reaching the nominal pressure value without further turning the nut or the screw.